Low Power Conversion Efficiency Research Articles

Comparative Analysis of the Stability and Performance of Double-, Triple-, and Quadruple-Cation Perovskite Solar Cells for Rooftop and Indoor Applications.

The solar energy market is predicted to be shared between Si solar cells and third-generation photovoltaics in the future. Perovskite solar cells (PSCs) show the greatest potential to capture a share there as a single junction or in tandem with silicon. Researchers worldwide are looking to optimize the composition of the perovskite film to achieve an optimal bandgap, performance, and stability. Traditional perovskites have a mixture of formamidinium and methyl ammonium as the A-site cation in their ABX3 structure. However, in recent times, the use of cesium and rubidium has become popular for making highly efficient PSCs. A thorough analysis of the performance and stability of double-, triple-, and quadruple-cation PSCs under different environmental conditions was performed in this study. The performance of the device and the films was analyzed by electrical measurements (J-V, dark J-V, EQE), scanning electron microscopy, atomic force microscopy, photoluminescence, and X-ray diffraction. The quadruple-cation device with the formula Cs0.07Rb0.03FA0.77MA0.13PbI2.8Br0.2 showed the highest power conversion efficiency (PCE) of 21.7%. However, this device had the least stability under all conditions. The triple-cation device with the formula Cs0.1FA0.6MA0.3PbI2.8Br0.2, with a slightly lower PCE (21.2%), was considerably more stable, resulting in about 30% more energy harvested than that using the other two devices during their life cycle.

Simulation and optimization of 30.17% high performance N-type TCO-free inverted perovskite solar cell using inorganic transport materials

Perovskite solar cells (PSCs) have gained much attention in recent years because of their improved energy conversion efficiency, simple fabrication process, low processing temperature, flexibility, light weight, and low cost of constituent materials when compared with their counterpart silicon based solar cells. Besides, stability and toxicity of PSCs and low power conversion efficiency have been an obstacle towards commercialization of PSCs which has attracted intense research attention. In this research paper, a Glass/Cu2O/CH3NH3SnI3/ZnO/Al inverted device structure which is made of cheap inorganic materials, n-type transparent conducting oxide (TCO)-free, stable, photoexcited toxic-free perovskite have been carefully designed, simulated and optimized using a one-dimensional solar cell capacitance simulator (SCAPS-1D) software. The effects of layers’ thickness, perovskite’s doping concentration and back contact electrodes have been investigated, and the optimized structure produced an open circuit voltage (Voc) of 1.0867 V, short circuit current density (JSC) of 33.4942 mA/cm2, fill factor (FF) of 82.88% and power conversion efficiency (PCE) of 30.17%. This paper presents a model that is first of its kind where the highest PCE performance and eco-friendly n-type TCO-free inverted CH3NH3SnI3 based perovskite solar cell is achieved using all-inorganic transport materials.

Work Function Tuning of Carbon Electrode to Boost the Charge Extraction in Hole Transport Layer-Free Perovskite Solar Cells.

Perovskite solar cell (PSC) is a promising photovoltaic technology that achieves over 26% power conversion efficiency (PCE). However, the high materials costs, complicated fabrication process, as well as poor long-term stability, are stumbling blocks for the commercialization of the PSCs in normal structures. The hole transport layer (HTL)-free carbon-based PSCs (C-PSCs) are expected to overcome these challenges. However, C-PSCs have suffered from relatively low PCE due to severe energy loss at the perovskite/carbon interface. Herein, the study proposes to boost the hole extraction capability of carbon electrode by incorporating functional manganese (II III) oxide (Mn3O4). It is found that the work function (WF)of the carbon electrode can be finely tuned with different amounts of Mn3O4 addition, thus the interfacial charge transfer efficiency can be maximized. Besides, the mechanical properties of carbon electrode can also be strengthened. Finally, a PCE of 19.03% is achieved. Moreover, the device retains 90% of its initial PCE after 2000 h of storage. This study offers a feasible strategy for fabricating efficient paintable HTL-free C-PSCs.

Designing of phenothiazine dioxide based donor molecules with improved photovoltaic parameters for efficient perovskite solar cells

Perovskite solar cells are the emerging photovoltaics devices due to their high efficiency because of broad range light absorption and better charge carrier and separation properties. However, in the past few decades their material degradation and instability in the device operation accompanying low power conversion efficiency were the major drawbacks. To address these shortcomings, here we designed five Phenothiazine based novel hole-transporting molecules of d-Pi-A type with improved photovoltaic parameters for efficient perovskite solar devices. Detailed DFT calculation analysis with MPW1PW91 functions and 6–31 G basis set were performed for all molecules. Structural geometrical analysis along with density of states and transition density matrix analysis were extensively performed. Their electronic properties including HOMO and LUMO energies including energy gap, electron affinity, and ionization potential were also explored to elucidate the charge transfer characteristics. The lower values of reorganization in relation to reference molecules show that all designed molecules are better hole-transporters for perovskite building blocks. Lastly the devices photovoltaic parameter calculations for all the designed systems revealed that their Fill factor (FF), VOC and resultant power conversion efficiencies were significantly improved (18.63–22.81 % than its reference counterpart with PCE of 16.7 %) demonstrating their great potential for the future efficient PSCs devices.

Polyvinyl Chloride-Based Luminescent Downshifting Film with High Flame Retardancy and Excellent UV Resistance for Silicon Solar Cells.

Solar power generation, as a clean energy source, has significant potential for development. This work reports the recent efforts to address the challenge of low power conversion efficiency in photovoltaic devices by proposing the fabrication of a luminescence downshifting layer using polyvinyl chloride (PVC) with added fluorescent dots to enhance light utilization. A photoluminescent microsphere (HCPAM) is synthesized by cross-linking hexachlorocyclotriphosphazene, 2-iminobenzimidazoline, and polyethyleneimine. Low addition of HCPAM can improve the fire safety of PVC films, raising the limiting oxygen index of PVC to 32.4% and reducing the total heat release and smoke production rate values by 14.5% and 42.9%, respectively. Additionally, modified PVC film remains a transparency of 88% and shows down-conversion light properties. When the PVC+1%HCPAM film is applied to the solar cell, the short-circuit current density increases from 42.3 to 43.8mAcm-2, resulting in a 7.0% enhancement in power conversion efficiency. HCPAM also effectively delays the photooxidative aging of PVC, particularly at a 3% content, maintaining the surface morphology and optical properties of PVC samples during ultraviolet aging. This study offers an innovative strategy to enhance the fire and UV-resistant performance of PVC films and expand their applications in protecting and efficiently utilizing photovoltaic devices.

BODIPY-Based Macrostructures: A Design Strategy toward Enhancing the Efficiency of Dye-Sensitized Solar Cells.

Among the metal-free dyes, boron dipyrromethene (BODIPY) has attracted much attention in the solar cell industry due to its thermal stability and tunable electronic and photophysical properties. However, the low power conversion efficiency of dye-sensitized solar cells based on BODIPY has limited their widespread application. Accordingly, different types of structural modifications have already been proposed to improve the photophysical properties of the BODIPY dyes. In this study, we used the strategy of constructing BODIPY-based covalent macrostructures by integrating two BODIPY subunits via a π-linker in linear and cyclic configurations. To this end, various types of the π-linkers including butadiyne, phenyl, and thiophene derivatives are considered. The structural, electronic, and optical properties as well as the photovoltaic performance of BODIPY dimers are theoretically calculated within DCM solvent. The results indicate that for a given linker, the BODIPY dimers with a linear configuration show better performance as compared to their macrocyclic counterparts. The reason is the enhancement of π-conjugation length, higher light harvesting ability, and proper charge carrier separation in linearly linked BODIPYs. In the cyclic series, the dyes incorporating phenyl linkers exhibit greater power conversion efficiency of up to 9%. For the dyes with a linear configuration, the involvement of a thienyl-thiophene bridge results in lower charge recombination and enhances the efficiency by up to 15%, which are expected to be potential candidates for organic dyes applied in DSSCs.

Synergistically manipulating the shape of alkyl-chain and asymmetric side groups of non-fullerene acceptors enables organic solar cells to reach 18.5% efficiency

Side-chain modification and asymmetric design for non-fullerene acceptors (NFAs) have been proven to be effective methods for harvesting high-performance organic solar cells (OSCs). Combining the two molecular design strategies, we adopted phenyl chain and alkyl chains with different shapes to develop two novel asymmetric NFAs, named BTP-P2EHC11 and BTP-P2EHC2C4. Compared with BTP-P2EHC2C4 attached 2-ethylhexyl side chain, BTP-P2EHC11 with linear alkyl side chain have slightly red-shifted absorption and intensive absorption strength. Moreover, the PM6:BTP-P2EHC11 blend film presents higher and more balanced charge mobilities, reducing charge recombination, tighter intermolecular packing and more favorable fibrous network morphology with appropriate phase separation than PM6:BTP-P2EHC2C4, which lead to significantly enhanced short-circuit current density (JSC) of PM6:BTP-P2EHC11-based devices. Thus, the OSCs based on PM6:BTP-P2EHC11 achieve a superior power conversion efficiency (PCE) of 18.50% with a good trade-off among open-circuit voltage (VOC) of 0.876 V, JSC of 26.85 mA cm−2 and fill factor (FF) of 78.65%, while PM6:BTP-P2EHC2C4-based device exhibits a lower PCE of 17.49%. Our investigation elucidates that the combination of finely optimizing the shape of alkyl-chain and asymmetric side groups of NFAs could pave a promising avenue toward morphology optimization and performance promotion of OSCs.

Environmental performance analysis of textile envelope integrated flexible photovoltaic using life cycle assessment approach

The textile envelope integrated flexible photovoltaic (TE-FPV) system is an emerging technology to promote building sustainability due to its lightweight structure, textile recovery easily, and renewable energy production. We develop six novel TE-FPV prototypes to determine their advantages of environmental performance. Especially, the systems with the solar cells directly printed on the textile are designed to show the promising sustainability prospect. Their environmental performances are conducted by a cradle-to-cradle life cycle assessment, and the results are compared to traditional building integrated photovoltaic (BIPV) technologies. The life cycle inventories of some textiles and solar cells are the first time to be developed. Based on the inventories, life cycle impact assessment is implemented to figure out environmental impacts in various categories. Furthermore, uncertainty analyses and sensitivity analyses are performed to unmask the effects of parameters on the fluctuation of environmental indicators. The results present that most of TE-FPV systems perform more sustainable and more recyclable over the traditional BIPV systems. The organic solar cell is unfavorable for the TE-FPV systems due to its low power conversion efficiency and short lifespan. ETFE facade integrated perovskite solar cell systems can refund the investment of greenhouse gas emission even on the north orientation, showing vast prospective potential in environmental performances. This work contributes significantly required environmental quantifications to promote the advancement of TE-FPV systems.

Buried interface modification toward efficient perovskite solar cell based on PVK hole transport layer

Poly (9-vinylcarbazole) (PVK) is an attractive hole transport material for organometallic-halide perovskite solar cell (PSC) because of its excellent hole transport ability and great film-forming property. Nowadays, the PVK hole-transport-layer (HTL) based PSC suffered from low power conversion efficiency (PCE), which mainly caused by bad interface contact between PVK and perovskite film, including the energy level mismatch. Doing PVK is an effective and facial method to modify the buried interface of perovskite film. In this work, 1,1-bis [(di-4-tolylamino)phenyl]cyclohexane (TAPC) was added into PVK HTL. Experimental results show that a highest occupied molecular orbital (HOMO) level elevation was achieved after doping TAPC. Meanwhile perovskite films with increased crystallinity, reduced defect density and enhanced light absorbance were obtained. Finally, the PCE of PSCs were enhanced from 10.81% of the control device to 15.65% of PVK + 40% TAPC device. The reason that responsible for the energy level lifting, morphology improvement and performance enhancement were thoroughly investigated and analyzed. This work provides an alternative option for the dopant in PVK to improve the buried interface of perovskite film, and therefore, the performance of PSCs.

Spray-Coated MoO3 Hole Transport Layer for Inverted Organic Photovoltaics.

This study focuses on the hole transport layer of molybdenum trioxide (MoO3) for inverted bulk heterojunction (BHJ) organic photovoltaics (OPVs), which were fabricated using a combination of a spray coating and low-temperature annealing process as an alternative to the thermal evaporation process. To achieve a good coating quality of the sprayed film, the solvent used for solution-processed MoO3 (S-MoO3) should be well prepared. Isopropanol (IPA) is added to the as-prepared S-MoO3 solution to control its concentration. MoO3 solutions at concentrations of 5 mg/mL and 1 mg/mL were used for the spray coating process. The power conversion efficiency (PCE) depends on the concentration of the MoO3 solution and the spray coating process parameters of the MoO3 film, such as flow flux, spray cycles, and film thickness. The results of devices fabricated from solution-processed MoO3 with various spray fluxes show a lower PCE than that based on thermally evaporated MoO3 (T-MoO3) due to a limiting FF, which gradually increases with decreasing spray cycles. The highest PCE of 2.8% can be achieved with a 1 mg/mL concentration of MoO3 solution at the sprayed flux of 0.2 mL/min sprayed for one cycle. Additionally, S-MoO3 demonstrates excellent stability. Even without any encapsulation, OPVs can retain 90% of their initial PCE after 1300 h in a nitrogen-filled glove box and under ambient air conditions. The stability of OPVs without any encapsulation still has 90% of its initial PCE after 1300 h in a nitrogen-filled glove box and under air conditions. The results represent an evaluation of the feasibility of solution-processed HTL, which could be employed for a large-area mass production method.

Controlled Fabrication of Composite Transparent Electrodes for Semitransparent Perovskite Solar Cells with above 84% Fill Factor

In the quest to develop efficient semitransparent perovskite solar cells (ST‐PSCs), the primary challenge lies in transparent electrode fabrication. n–i–p‐type ST‐PSCs often suffer from low fill factors (FF) and power conversion efficiency (PCE), attributed to the undesirable buffer layer/transparent conductive oxide combination of the transparent electrode. In this context, this study proposes an efficient composite transparent electrode (CTE) system comprising an ultrathin MoO3 buffer layer (5 nm) and low‐power sputtered indium zinc oxide (45 W–220 nm). The electrode demonstrates excellent compatibility with Spiro‐OMeTAD‐based devices, effectively addressing the low FF and PCE challenges of ST‐PSCs. Employing the new CTE, the best‐performing ST‐PSCs with the absorption edge at 1.57 eV achieve a remarkable PCE of 21.96% with an FF of up to 83.80%. More transparent ST‐PSCs with thickness‐modulated absorbers and average transmittance of 50.77% at 600–1200 nm wavelength result in a PCE of up to 20.91% with the highest known FF of 84.02%. This study presents a general and viable approach for fabricating efficient ST‐PSCs.

Exploring the influence of pressure-induced semiconductor-to-metal transition on the physical properties of cubic perovskites FrXCl3 (X = Ge and Sn)

Even though lead halide perovskites have outstanding physiochemical properties and improved power conversion efficiency, most of these compounds threaten their future commercialization because of their instability and highly toxic nature. Thus, it is preferable to use stable alternative elements rather than lead to make environmentally friendly perovskite material that will have comparable optical and electronic properties to those constructed from Pb-based perovskites. However, devices constructed from lead-free perovskites typically display a lower power conversion efficiency. Applying hydrostatic pressure could be deemed an effective method to alter the physical properties of these compounds. This not only improves their performance in application but also reveals significant correlations between structure and properties. This work uses DFT to investigate the structural, electronic, optical, and elastic properties of non-toxic, francium-based halide perovskites FrXCl3 (X = Ge, Sn) at different levels of hydrostatic pressures that vary from 0 to 10 GPa. The estimated structural parameter's strong correlation with the data from earlier studies ensures the accuracy of the current findings. Pressure causes the Fr−Cl and Ge (Sn)–Cl bonds to shorten and become stronger. The electronic property calculations demonstrated that both compounds are direct band-gap semiconductors. The application of pressure leads to a linear reduction in the band gap (semiconducting to metallic state) and raises the electronic density of states around the Fermi level by forcing the valence band electrons upward, indicating that the optoelectronic device's performance can be tuned and improved. The values of the dielectric constant, absorptivity and reflectivity showed an increasing tendency with pressure. As the pressure applied to the compounds increases, the absorption spectra show a redshift. These findings suggested that the FrXCl3 (X = Ge and Sn) compound becomes more appropriate for usage in optoelectronic applications under pressure. Furthermore, our examination of the mechanical properties indicates that both FrGeCl3 and FrSnCl3 exhibit mechanically stability, and ductility. Interestingly, we observe an increase in ductility as pressure levels rise.

Electron injection and defect passivation for high-efficiency mesoporous perovskite solar cells.

Printable mesoscopic perovskite solar cells (p-MPSCs) do not require the added hole-transport layer needed in traditional p-n junctions but have also exhibited lower power conversion efficiencies of about 19%. We performed device simulation and carrier dynamics analysis to design a p-MPSC with mesoporous layers of semiconducting titanium dioxide, insulating zirconium dioxide, and conducting carbon infiltrated with perovskite that enabled three-dimensional injection of photoexcited electrons into titanium dioxide for collection at a transparent conductor layer. Holes underwent long-distance diffusion toward the carbon back electrode, and this carrier separation reduced recombination at the back contact. Nonradiative recombination at the bulk titanium dioxide/perovskite interface was reduced by ammonium phosphate modification. The resulting p-MPSCs achieved a power conversion efficiency of 22.2% and maintained 97% of their initial efficiency after 750 hours of maximum power point tracking at 55 ± 5°C.

Space–Charge‐Limited Photocurrent as a Possible Cause for Low Power Conversion Efficiency in GaInN/GaN‐Based Optoelectronic Semiconductors

This study attempts to understand the cause of low power conversion efficiency (PCE) in III‐nitride optoelectronic semiconductors under optical operation. For this purpose, a GaInN/GaN heterojunction semiconductor is fabricated, and the photoexcited current–voltage (PEJV) curves are carefully measured depending on the optical excitation power and temperature. The results show unexpected excitation power‐ and temperature‐dependent behaviors, that is, the PCE decreases with increasing excitation power and increases with increasing temperature. To understand this, the space–charge‐limited photocurrent (JPh,SCL) theory (also referred to as Goodman and Rose theory) is employed, where the accumulated charge carriers in the active layer play a significant role. The conduction of JPh,SCL is ascertained by analyzing the PEJV curves. The conduction of JPh,SCL is investigated as a possible cause of the low PCE, revealing that the conduction of JPh,SCL could limit the high‐power operation of the device.

Advances in Organic Photovoltaic Cells: Fine‐Tuning of the Photovoltaic Processes

This work highlights recent advancements in how the structures and chemical makeups of the active layer materials affect photovoltaic processes and performance in terms of power conversion efficiency and stability. It further sheds light on the performance optimization of organic photovoltaic cell (OPV) and the relationship between these optimization conditions and OPVs performance. The use of different substituents on the same donor or acceptor material has different optimal conditions. Furthermore, it is shown that the addition of different third components in the active layer has different optimal concentration points. This review also highlights and suggests a possible way to improve the stability of OPV through modification of the active layer. To date, some studies showed that incorporation of the third component in the active layer leads to over 97% stability after more than 1000 h.

Lead-free perovskite solar cell based on methyl ammonium tin iodide: Possible power conversion efficiency enhancement by device simulation

Although solar cells have the potential to create an endless amount of electrical power, their comparatively low power conversion efficiency draws the curiosity of both academics and industry. The primary goal of this research is to examine the performance of lead-free perovskite solar cells that use methyl ammonium tin iodide (CH3NH3SnI3) as the active material. The SCAPS-1D programme is employed for the simulation and analysis of the solar cells. The solar cell structure was analyzed computationally, and the resulting output was examined by comparing the I-V characteristic curves across different combinations of parameters. Various materials and combinations of parameters were explored in an effort to enhance the power conversion efficiency of solar cells based on CH3NH3SnI3. A series of comprehensive evaluations were undertaken to examine the influence of technical factors, including absorber layer thickness, defect density, and acceptor density, on performance. The acceptor density that yielded the best results was selected in order to improve the performance of the solar cell. The simulation results were acquired across a temperature range spanning from 300K to 450K, revealing enhanced stability within the range of typical ambient temperatures. The study produced encouraging findings, which included a short circuit current density of 30.81mA/cm2, an open circuit voltage of 1.25V, a fill factor of 80.35%, and a power conversion efficiency of 31.11%. This study has the potential to facilitate the development of environmentally sustainable, highly efficient, and cost-effective thin film solar cells based on CH3NH3SnI3, which exhibit noteworthy physical characteristics as alternatives to lead-containing perovskite solar cells.

Development and Challenges of Large‐Area All‐Perovskite Tandem Solar Cells and Modules

The efficiency of all‐perovskite tandem solar cells has recently surpassed that of single‐junction perovskite solar cells, showing great potential as a future photovoltaic technology due to its low manufacturing cost and high power conversion efficiency potential, yet the size of these cells is still at the laboratory level. It is highly required to develop scalable preparation methods to fabricate large‐area all‐perovskite tandem solar modules for commercial applications. Herein, the key challenges encountered in the laboratory of all‐perovskite tandem solar cells and the existing solutions are summarized and some views on the preparation of large areas and modules are given.

Regulating the Quantum Dots Integration to Improve the Performance of Tin–Lead Perovskite Solar Cells

AbstractDespite their advantageous attributes, such as a narrow bandgap and reduced toxicity, tin–lead halide perovskites (TLHPs) have received limited attention due to their lower power conversion efficiency (PCE) relative to lead‐only variants. In this study, a transformative approach is introduced that leverages perovskite quantum dots (PQDs) to optimize TLHP solar cells. While conventional oleyl‐capped PQDs enhance the open circuit voltage (VOC), the long‐chain ligands hinder charge transport. To overcome this limitation, a post‐treatment with isopropyl alcohol effectively dissociates these ligands and PQD crystals, resulting in reduced defect density, improved charge transfer, and elevated quasi‐Fermi level splitting in the TLHP device. Consequently, the PCE of the device is notably increased from 19.0% to 23.74% and elevated the VOC from 0.78 to 0.87 V, without compromising the photocurrent or fill factor. The findings highlight PQD modification as a compelling avenue for TLHP solar cell enhancement, particularly in boosting VOC.

Mitigation of Exciton Quenching Sites in All‐Metal‐Oxide‐Based Transparent Photovoltaic

Transparent photovoltaic (TPV) devices offer the potential to generate power without being visible to the human eye, making them ideal for use in building integrated photovoltaic applications. TPV devices based on inorganic materials offer eco‐friendly frameworks with stable performances. However, their low power conversion efficiency and incapacity to produce onsite power for real‐time practical applications limit their widespread deployment. Mitigation of exciton quenching by reducing the interface and bulk recombination can significantly improve the performance of TPVs. To address this, the present study investigates the effect of passivation layers (PLs) in all‐metal‐oxide TiO2/Cu2O TPV. The TiO2/Cu2O TPV interface is passivated by depositing thin Al2O3 and Ga2O3 films. The study comprehensively analyzes how passivation controls the interfacial and bulk defects. Electrochemical impedance spectroscopy and X‐ray photoelectron spectroscopy are used to elucidate exciton quenching mechanisms. The results show that the insertion of a thin PL on top of TiO2 effectively mitigates exciton quenching by suppressing hydroxyl ions, Ti2+, and Ti3+ bulk states. Ga2O3 PL, in particular, leads to a substantial enhancement in short‐circuit current density (12.4 mA cm−2) and open‐circuit voltage (669 mV). The study also demonstrates the real‐time onsite energy production and its utilization through the operation of an electric fan.

Comprehensive Insight into the Structure Contribution of A2‐A1‐D‐A1‐A2 Acceptor to Performance of P3HT Solar Cells

AbstractPoly(3‐hexylthiophene) (P3HT) acting as one of the most popular and low‐cost polymers is quite suitable for commercialization of organic solar cells but suffers from low power conversion efficiency (PCE) because of the limited matching non‐fullerene acceptors (NFAs). One important reason that restricts the enhancement but remains unresolved is the undisclosed contributions of the subtle structure modification to the obvious performance change. In combination with the previous reports and this work, herein A2‐A1‐D‐A1‐A2 type NFAs with single, dual, and triple modifications based on parent BTA3 is designed, including benzyl‐substitution on A2 group (namely Bn modification), fluorine‐substitution on A1 group (namely F modification), and thieno[3,2‐b]thiophene‐substitution on D group (namely TT modification). It is finally found that the binary devices of P3HT with these NFAs underwent unexpected variations in the aspect of molecular optoelectronic property, blend morphological feature and charge generation process. The triple modification (including Bn, F, and TT) gives full play to their unique advantages and consequently increases PCE by 60%. To the knowledge, the obtained optimal PCE is one of the highest values for A2‐A1‐D‐A1‐A2 type NFAs. This study provides clearer insights into the rational substitutions on the A2‐A1‐D‐A1‐A2 acceptors matched with P3HT for high‐performance organic photovoltaics.